PITTSBURGH - The mother and grandparents of a Pennsylvania boy, who authorities say was starved nearly to death, are facing criminal charges, according to court documents.

The 7-year-old boy weighed less than 25 pounds (11 kg) when he was found last month at a home in Greenville in western Pennsylvania by child welfare workers investigating a report of "a boy who looked like a human skeleton," according to an affidavit obtained on Monday.

"He was being starved in his own home," Dr Jennifer Wolford of the University of Pittsburgh Children's Hospital Child Advocacy Center said in the affidavit.

"He was within a month of having a major cardiac event that he probably would not have recovered from," she said, calling it "the worst case of medical neglect that I have ever seen in my seven years as a pediatrician."

According to the criminal complaint, the boy was home schooled and isolated since August 2013 and was not allowed out of the house except for the back porch, where he would catch bugs and eat them.

Investigators interviewed his two normal-weight sisters, ages 11 and 4, and determined that the boy had been singled out for abuse, the affidavit said. A 9-year-old brother also was underweight but not as severely, it said.

The boy was fed small portions of tuna fish and eggs and beaten if he was caught sneaking food, usually peanut butter and bread, it said.

At times he was forced to sleep in the basement and only allowed to bathe as punishment in an icy shower. According to the affidavit, when asked to shower at the hospital, he asked if the water would be cold.

Grandparents Dennis and Deana Beighley and the boy's mother, Mary Rader, each were charged late last week with assault, unlawful restraint of a minor, false imprisonment and endangering the welfare of children.

The children have been placed in foster care, according to the Mercer County District Attorney's Office.

The adults were scheduled in court on July 30 for a preliminary hearing.

The boy has gained at least 20 pounds (nine kg), but doctors say it will take one to two years for him to reach a normal weight.

"It is impossible to know what his long-term intelligence will be or how it may have suffered," Wolford said in the affidavit.